1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor wafer by mirror polishing its surfaces.
2. Description of the Related Art
A conventional method for fabricating a semiconductor wafer by mirror polishing the surface, as shown in FIG. 6, includes the following steps.
(1) Referring to FIG. 7A, a wafer is sliced from a single-crystal silicon ingot by an inner diameter saw slicing step.
(2) Referring to FIG. 7B, the rim of the silicon wafer is chamfered to prevent from cracking.
(3) Referring to FIG. 7C, the sliced surfaces of the chamfered wafer are lapped to attain a consistent thickness of the wafers.
(4) Deformation caused by the lapping process is removed by an etching process, as shown in FIG. 7D.
(5) Referring to FIG. 7E, the wafer is polished and mirror polished after the etching process.
(6) After the mirror polishing process, the wafer is cleaned with a chemical solution.
In order to increase the manufacturing efficiency, chemical solutions of high etching rates, such as mixed acid, are utilized in the aforementioned etching step.
However, the chemical solution etches the wafer so fast that the rim portion thereof tends to be over-etched. Referring to FIG. 7D, the center portion is therefore thicker than the rim portion of the wafer after the etching process. A wafer after the single-side polishing process still has a thickness difference of .DELTA.t due to the thickness variation in the fast etching process.
It is difficult to control the thickness difference .DELTA.t under 1 .mu.m when etching solutions such as HF, HNO.sub.3, CH.sub.3 COOH and H.sub.2 O are used.
Some methods, for example, that disclosed in Japanese Patent Publication No. 6-61681, entitled "Mirror Polishing Fabrication Method," have been provided to overcome the problem of thickness variation.
The mirror polishing fabrication method in which single or double sides of the wafer are polished has the single or double sides ground before a final polishing process.
However, the mirror polishing method introduces an additional single- or double-side polishing process into the conventional fabrication method thereby making the process more complicated.
Moreover, the grinding process on the wafer will result in processing deformation. In order to remove the deformation, other etching and polishing steps are required, thus further complicating the process.